It has become increasingly common to integrate devices with different characteristics on a single IC. The different types of devices have different operating and breakdown voltages thus requiring multiple gate dielectric layers (e.g. oxide layers) having different thicknesses to be formed. For example, the IC may have low, medium and high voltage device regions with the gate dielectric associated with the FET in the high voltage region being the thickest, the gate dielectric associated with the FETs in the low voltage device region being the thinnest, and the gate dielectric associated with the medium voltage device region being somewhere between the thinnest and the thickest. ICs having FETs formed as such are referred to as triple gate oxide (TGO) chips, and are referred to more generally as triple gate dielectric (TGD) chips.
In order to form two or more different gate dielectric thicknesses in the respective device regions of an IC, several dielectric removal and deposition or growth steps are generally necessary. However, when integrated into a trench isolation process comprising flow, as described below, the dielectric removal processes generally have a detrimental impact on the trench isolation comprising structures which are commonly used for electrical isolation between adjacent FETs and between certain regions of individual FETs.
As used herein, the term “trench isolation” applies for both conventional (e.g. bulk Si) substrates as well as silicon on insulator (SOI) substrates. Applied to conventional substrates, as used herein the term trench isolation includes deep trench isolation which is typically 1-5 μm deep, and shallow trench isolation which is typically <1 μm deep, such as 0.3 to 0.7 μm deep. Applied to SOI substrates, as used herein, trench isolation includes the isolation regions between the active area islands. In the case of thin film SOI, the trench isolation regions like in the conventional substrate case are generally filled with a deposited dielectric, but are typically shallower that their conventional substrate counterparts, being generally <0.5 μm deep, such as 0.01 to 0.3 μm deep.
Following formation by a generally anisotropic etch, the resulting trench is generally first lined with a thin thermal “liner” oxide and then filled with a deposited dielectric, such as a plasma enhanced oxide. As known in the art, deposited oxides generally have a higher etch rate as compared to the thermally grown oxides which are generally used as gate dielectrics. As a result, a wet etch (e.g. dilute HF deglaze) of a gate dielectric in the non-gate areas after trench formation generally results in a significant loss in trench dielectric thickness, with a larger loss (e.g. about 1.5 to 2 times) being at the peripheral edges of the trench isolation regions which etch at a higher rate as compared to the trench dielectric away from the peripheral edges. The trench edges thus become recessed, giving rise to what are commonly referred to as divots.
The divots in the trench isolation structures are undesirable as they increase the sub-threshold leakage current of FETs in the adjacent active semiconductor regions. This effect can be particularly significant for the low voltage regions having thin gate dielectric comprising FETs. In addition, as known in the art, relatively thick gate oxides (e.g. >300 Angstroms) grow non-uniformly being thicker away from the trench edge and being thinner (typically ≦75% of the thicker thickness) at or near the trench/active area edges/corners. This corner thinning results in processes targeting thick oxide to be thicker than otherwise necessary to avoid gate dielectric breakdown at the trench corner. Furthermore, sharpening at the trench corner can cause high stress levels that can result in crystal defects in the active area that can reduce circuit yield. Accordingly, it is desirable to provide a method for fabricating a multiple gate dielectric chip in which divot formation and corner sharpening at the trench corners is significantly reduced.